1. Field of the Invention
The invention relates to a method and to an installation for making a workpiece comprising at least one tubular part obtained by bending a metal sheet. This method and this installation are particularly adapted to the manufacture of a shadow mask support frame for a cathode-ray display tube.
2. Brief Description of the Related Art
Such a frame is used for example in the domain of the manufacture of colour televisions. The colour display cathode-ray tubes comprise a metal foil pierced with a plurality of holes or slots, called “shadow mask” and disposed between the electron gun and the display screen. Such a shadow mask is supported by a frame, generally rectangular in shape, which holds it in position in the vicinity of the display screen and, if necessary, ensures that the mask is taut in order to limit the deformations resulting from the local overheatings created by the electron beams.
It is known from EP-A-0 809 272 to produce a shadow mask frame from two thin metal strips joined to each other and comprising ribs for rigidification. Such a frame is light and rigid, which allows it to be used for a taut shadow mask and for a crimped shadow mask. Application FR-A-2 790 140 discloses a shadow mask frame comprising tubular parts formed by bending a metal sheet. The sheet may be bent by different methods, in particilar by hand, but a manual bending is not compatible with high-rate industrial production. Furthermore, the known methods of bending with the aid of machines comprising an apron, sometimes called “support strap”, on which a metal sheet to be bent is held with the aid of a holding clamp, and an articulated bending flap, do not allow tubular parts to be easily manufactured.